BOSTON — Francis P. Salemme, 84, arrived in federal court this week in a wheelchair, his hunched frame shrouded in a loose suit and his feet tucked into easy black sneakers. He gripped the arms of a wooden chair behind the defense table, gingerly rising for the jury’s arrival. His slight face, papery skin and wispy gray hair were a startling departure from his mug shots of long ago, back when his jaw was set like concrete.

It can be hard to absorb that this frail man used to be known as Cadillac Frank, a fearsome gangster who admitted to multiple killings, went to prison for a car bombing that blew a man’s leg off, and survived an assassination attempt outside an International House of Pancakes. He was once a powerful mafia boss, the head of the New England family of La Cosa Nostra, the authorities say, and a contemporary of James “Whitey” Bulger, the notorious Boston crime boss.

Not long ago, human remains turned up behind a mill building in Providence, R.I., setting into motion this new murder trial against Mr. Salemme. But the authorities say the crime itself took place a quarter-century ago. And most everything feels like a flashback in the trial that began this week, including the who’s who of underworld players trudging into court in sensible shoes. It is a reminder that it has been a long time since a clear-cut set of larger-than-life gangsters controlled New England’s criminal underworld.

Anthony Cardinale, a defense lawyer who has represented mobsters — including, decades ago, Mr. Salemme — described the trial here as a “last vestige” of such federal prosecutions. “Everybody’s been burned to a crisp here by informants,” he said.

These days, organized crime in New England is “in a continuous state of uncertainty and disarray because of so many leadership changes,” said Brendan Doherty, the former superintendent of the Rhode Island State Police. “It’s not what it was 20 to 25 years ago, but there’s no one trial that’s going to put an end to it.”

“The new young criminals coming in — they don’t even know who these old-time mobsters are,” Mr. Doherty said.

The trial is unfolding in a city that is in some ways far different from the one where Mr. Salemme and other bosses once held so much sway. South Boston, the home of Mr. Bulger, has been transformed by the arrival of young professionals. Even The Channel, a squat nightclub that the authorities say Mr. Salemme was involved in 25 years ago, is long gone. A glassy new headquarters for General Electric has broken ground in the area.

Prosecutors say that back in 1993, Mr. Salemme and a son, Frank Salemme Jr., had a secret stake in The Channel, which was managed by a real estate developer, Steven DiSarro. The Salemmes worried that Mr. DiSarro might help investigators build a case against them, the prosecutors say. They say Mr. Salemme stood by as his son strangled Mr. DiSarro while another associate, Paul M. Weadick, held his legs, then had him buried in Providence. The younger Mr. Salemme died years ago; the other two men now stand charged with one count of murdering a witness.

The two men have pleaded not guilty and their lawyers called the government’s witnesses “accomplished liars with a history of lying.” Steven Boozang, a lawyer for Mr. Salemme, said his client has admitted to gangland killings, but not to this one. “It was a little bit of kill or be killed back then,” Mr. Boozang said of Mr. Salemme, who previously pleaded guilty to racketeering. “Just because he’s done these bad things doesn’t mean he’s done this.”

Mr. Salemme helped federal prosecutors in the early 2000s by testifying against a corrupt F.B.I. agent enmeshed in Mr. Bulger’s world, and had been living a quiet existence in a government witness protection program, prosecutors said, when Mr. DiSarro’s bones were discovered in Providence in 2016. When the bones appeared, Mr. Salemme took off, and the authorities caught up with him in Connecticut, where his car was found to contain $28,000 in cash, prosecutors said.

“These are the remains of the marquee, the ‘glory years’ of organized crime in Boston, whether it was the mafia or the Bulger gang,” said Dick Lehr, an author and a professor of journalism at Boston University. “You had a host of marquee names and players and Cadillac Frank was one of them. No one’s emerged in the past 20 years to capture the public eye with that kind of swagger and notoriety.”

Prosecutors say they plan to call on an array of witnesses with complicated reputations and nicknames to testify against Mr. Salemme over the coming weeks. Among them: Stephen “The Rifleman” Flemmi, a well known confidante of Mr. Bulger, who is said to have walked in on the killing. And Robert “Bobby the Cigar” DeLuca, who is expected to testify about the burial of Mr. DiSarro.

On Thursday, Thomas Hillary, 73, an associate of Raymond L. S. Patriarca Sr., the onetime head of New England organized crime, described a bygone world from the stand.

“We were connected,” Mr. Hillary said, breezily recalling drug deals and rip-offs. At one point, he sputtered with anger as he said that Mr. Salemme had throttled him at a Chinese restaurant and run him out of town in 1990.